a) Field of the Invention
This invention relates to a production process of a carbonaceous material, and more specifically has as an object thereof the economical provision of a carbonaceous material assuring high performance, for example, as a negative pole material for lithium ion secondary batteries.
b) Description of the Related Art
In recent years, lightweight and compact designs have been rapidly adopted for electronic equipments and communication equipments. Keeping in step with this, a strong demand has also arisen for the size and weight reduction of secondary batteries which are used as their driving power supplies. Lithium ion secondary batteries having high energy density and high voltage have been proposed.
A lithium ion secondary battery uses, for example, lithium cobalite for a positive pole and a carbonaceous material such as graphite for a negative pole, so that lithium ions are occluded in the carbonaceous material during charging and these lithium ions are released from the negative pole upon discharging.
Lithium cobalite is used most widely as a positive pole material these days although a variety of materials have been proposed. As a negative pole material, on the other hand, carbon is used primarily. Examples of this carbon can be divided principally into graphitic crystalline materials and carbonaceous amorphous materials.
As one example of the graphitic crystalline materials, spheroidal fine particles of mesophase pitch are used as a negative pole material. These spheroidal fine particles of mesophase pitch are fine particles, which deposit in a spheroidal form in various pitches when the pitches are heated to temperatures of from 350.degree. C. to 500.degree. C. They have a graphite-like structure. If the above-described heat treatment of each pitch is continued as is, the whole pitch is eventually converted into coke. To obtain the above-described fine particles of mesophase pitch, it is therefore necessary to add a solvent to the heat-treated pitch in the course of the formation of spheroidal fine particles of mesophase pitch and then to separate the fine particles alone by centrifugation or a like method see, for example, Japanese Patent Application Laid-Open (Kokai) No. HEI 4-115458!.
The separation of the spheroidal fine particles of mesophase pitch is conducted from the high-viscosity molten pitch containing the solvent added therein, so that the separation requires cumbersome procedures and moreover, the amount of the spheroidal fine particles available per 100 parts by weight of the pitch used is as low as about 10 to 30 parts by weight or so, that is, the yield is very low. In addition, there is a problem of recovery of the solvent employed. Accordingly, for not only the cumbersome procedures but also the low yield, the resulting spheroidal fine particles of mesophase pitch is accompanied by the problem that they are extremely costly despite the use of the inexpensive raw material. This has become one of causes for the high cost of lithium ion secondary batteries as final products.
As a measure for overcoming the above-described problems, a process has been proposed for the provision of a carbonaceous material. According to this process, coal tar pitch is treated with quinoline to collect quinoline-soluble components. Further, toluene-insoluble components are removed from the quinoline-soluble components, whereby a so-called ".beta.-resin" is prepared. The ".beta.-resin" is hydrogenated and is then converted into a heavier resin, i.e., a mesophase pitch. Using this mesophase pitch as a raw material, the carbonaceous material is provided Japanese Patent Publication (Kokoku) No. SHO 64-33186 and Japanese Patent Application Laid-Open (Kokai) No. HEI 7-223808).
The above-described process is however accompanied by the problem that quinoline and toluene, which have high boiling points and high toxicity, are used in large amounts, although good spheroidal carbon powder can be obtained. Many unsolved problems therefore still remain, including handling and recovery problems of these solvents. As a consequence, there is the problem that the resulting spheroidal carbon material becomes considerably costly.